Layers of tin and tin alloys on electronic parts such as printed circuit boards, IC substrates and semiconductor wafers are used as solderable and bondable finishes in later manufacturing steps of such electronic parts.
The tin and tin alloy layers are usually deposited on metallic contact areas such as contact pads and bump structures. The contact areas are usually made of copper or copper alloys. In case such contact pads can be electrically contacted for deposition of tin and tin alloy layers such layers are deposited by conventional electroplating methods. However, in many cases the individual contact areas can not be electrically contacted. In such cases an electroless plating method needs to be applied. The method of choice in the industry for electroless plating of tin and tin alloy layers is immersion plating. The main disadvantage of immersion type plating is the limited thickness of the tin or tin alloy deposit. Immersion plating is based on an exchange between tin ions and the metallic copper contact area to be plated. With immersion type plating of tin or tin alloy layers the deposition rate decreases strongly with increasing tin layer thickness, since the exchange of Cu against Sn is hindered by the growing Sn layer. Therefore depositing tin layers exceeding a thickness of 1.0 μm with an immersion process may take too much time for many practical applications.
In situations where a thicker layer of tin or a tin alloy layer is required an autocatalytic type electroless plating process needs to be used. Plating bath compositions for autocatalytic plating of tin or tin alloys comprise a reducing agent.
The patent document U.S. Pat. No. 5,364,459 discloses electroless plating bath compositions for autocatalytic deposition of tin, the bath compositions comprising Ti3+ ions as reducing agent, a complexing agent such as a salt of citric acid or EDTA, Sn2+ and Pb2+ ions and carbonate ions as pH adjusting agent.
The patent document U.S. Pat. No. 5,360,471 discloses electroless plating bath compositions for solder deposition, the bath compositions comprising Ti3+ ions as reducing agent, complexing agents such as a salt of citric acid and EDTA, Sn2+ ions and ammonia as pH adjusting agent.
An autocatalytic tin plating bath consisting of TiCl3 as reducing agent, a salt of citric acid and EDTA as complexing agents, sodium acetate, Sn2+ ions, benzo sulfonic acid and ammonia as pH adjusting agent is disclosed in “Autocatalytische Zinnabscheidung” (“Autocatyltic Tin Deposition”), M. E. Warwick, B. F. Müller, Metall, Vol. 36, 1982, pp. 955 to 958.
An autocatalytic plating bath for deposition of tin comprising a water-soluble tin compound, a water-soluble titanium compound, an organic complexing agent, and an organosulfur compound selected from the group consisting of mercaptanes and sulfides is disclosed in WO 2009/157334 A1.
An autocatalytic tin plating bath comprising a water-soluble tin compound, a water-soluble titanium compound and an organic complexing agent containing trivalent phosphorous is disclosed in WO 2008/081637 A1.
The autocatalytic tin plating bath compositions known in the art suffer from a balance between prevention of undesired plate-out of tin and a sufficient deposition speed required for practical applications.